This invention relates generally to fuses, and, more particularly, to dual element fuses.
Fuses are widely used as overcurrent protection devices to prevent costly damage to electrical circuits. Typically, fuse terminals form an electrical connection between an electrical power source and an electrical component or a combination of components arranged in an electrical circuit. One or more fusible links or elements, or a fuse element assembly, is connected between the fuse terminals, so that when electrical current through the fuse exceeds a predetermined threshold, the fusible elements melt, disintegrate, sever, or otherwise open the circuit associated with the fuse to prevent electrical component damage.
One type of a dual element, time delay fuse includes a short circuit fuse element and an overload fuse element. The short circuit element typically is a conductive strip having a number of areas of reduced cross section, or weak spots. The weak spots are dimensioned to melt or otherwise open a circuit through the dual element fuse upon sustained predetermined overload current conditions, such as, for example, 700% of the current rating of the fuse. The overload fuse element, installed in series with the short circuit element, typically includes a spring-loaded trigger with a heating element. A fusing alloy, connects the heater elements to parts of the trigger and also connects the trigger to the short circuit fuse element. Upon sustained overload conditions, such as, for example, currents of 120% to 600% of the current rating of the fuse, the fusing alloy melts, thereby releasing a compression spring that separates the trigger from the short circuit fuse element and opens the electrical circuit through the fuse. In one such type of fuse, the trigger assembly includes a barrel surrounding the trigger and a resistive copper alloy heating strip supplying heat to the barrel for melting the fusing alloy of the trigger. Sec, for example, U.S. Pat. No. 5,239,291.
While the above-described dual element fuse construction is well suited for fuses having higher current ratings, for fuses of smaller current ratings, e.g., up to 10 amps, the heater strip becomes too thin and fragile for typical manufacturing operations. Resistive wires are sometimes used in lieu of the heater strips to supply heat to operate an overload fuse element trigger assembly upon the occurrence of sustained overload conditions. However, use of resistive wire to heat the trigger assembly conventionally requires a different, and more complicated construction of the trigger assembly in comparison to that described above. See for example, U.S. Pat. No. 4,888,573 employing a tension spring assembly for the trigger. Aside from the associated manufacturing difficulties of these trigger assemblies, resistance wire heating of the trigger in a dual element fuse does not always operate the trigger as effectively as desired. Still further, the trigger tends to undesirably increase watt losses for the circuit associated with the fuse, thereby reducing energy efficiency.
In one aspect, a dual element fuse is provided that includes a first conductive fuse coupler portion and an overload fusing assembly coupled to the first conductive fuse coupler portion. The overload fusing assembly includes a barrel having a flange at one end thereof, a trigger received within said barrel and positioned in a pre-operated position by a fusing alloy, and a conductive coil surrounding the barrel predominately in an area adjacent the flange. The conductive coil is connected between the first conductive fuse coupler portion and the flange, thereby concentrating heat generated in the conductive coil toward the flange. As such, the overload fusing assembly operates more efficiently with a simpler construction than known, lower amperage, overload fusing assemblies utilizing conductive wire to heat a trigger assembly.
In another aspect, a dual element fuse is provided that includes a first conductive fuse coupler portion, an overload fusing assembly coupled to the first fuse coupler portion, the overload fuse assembly comprising a barrel having a flange, a spring-loaded trigger mounted within the barrel in a pre-operated position, and at least one conductive coil surrounding the barrel and providing a conductive path between the first conductive coupler portion and the barrel flange. A short circuit fuse assembly is coupled to the trigger with a fusing alloy, and a second fuse coupler portion is coupled to the short circuit fuse assembly to complete a circuit through the fuse.
In still another aspect, an overload fusing assembly for a dual element fuse is provided. The overload fusing assembly includes a barrel comprising a longitudinal opening therethrough and a flange on an end thereof. The barrel flange includes at least one mounting aperture therein, and a trigger is received in the longitudinal opening and includes a flange located within the opening and a body extending from the opening in a pre-operated position. A spring is disposed between the barrel flange and the trigger flange, and the spring is in compression in said pre-operated position. A conductive wire is attached to the barrel flange and is wrapped around the barrel adjacent the barrel flange, thereby concentrating heat generated within said wire to the barrel near the flange.
In yet another aspect, an overload fusing assembly for a dual element fuse is provided. The overload fusing assembly includes a barrel comprising a longitudinal opening therethrough and a flange on an end thereof The flange includes at least one mounting aperture therein, and a rib extends on an external perimeter the barrel. A trigger is received in the barrel longitudinal opening and partially extends therefrom in a pre-operated position. A spring is disposed between the barrel flange and the trigger, and the spring is placed in compression in the pre-operated position. A conductive wire is attached to the barrel flange and is wrapped around the barrel between the barrel flange and the rib, thereby concentrating heat generated within the barrel to the barrel near the flange.